Norway rats (Rattus norvegicus) and roof rats (Rattus
rattus, also called black rats, ship rats) are different
species. A species, according to the biological species
concept, is a group of related individuals or populations that
can interbreed and produce fertile offspring. Members of different
species cannot produce fertile offspring together.

In rare circumstances, when two closely related species are kept
together in captivity, mating may occur. The resulting pregnancy may
be unsuccessful and the embryos die. Norway rat and roof rat crosses
are usually unsuccessful. Gray (1972) reports unsuccessful matings
between Norway and roof rats, and Chiasson (1980) records that Norway
and roof rats will not produce offspring, even if artificially
inseminated. Castle (1947) reports that crosses between R.
norvegicus and R. rattus are very difficult to obtain,
such that the embryos never come to term alive.

However, there is at least one anecdotal case of a cross between a
male R. norvegicus and three female R. rattus which
produced offspring, all of which were born alive but died shortly
after birth. The rats were owned by Jane Adamo of New York, and her
description of the event follows (pers. comm.):

On October 23, 1999, I adoped four "wild Norway baby
boys" from the AARK Wildlife Rehabilitation Center in
Pennsylvania. The animals had been abandoned by their
mother, hand raised from pinkies, then deemed unreleasable
wildlife. We estimated the ratlets had been born around
September 18, 1999. I installed them in a cage with Ick!, my
mature Norway male.

Over the next few weeks, I noticed a number of
irregularities. There was never any fighting between Ick!
and the babies; he loved them from the moment they were put
in the cage with him. In addition, the babies seemed
developmentally "stuck": they did not seem to be maturing,
"bulking up" as Norways do when they grow into adulthood.
Toward the end of December, the boys started getting
"peevish" and bitey.

On December 28, 1999, to my astonishment, one of the baby
"boys" had a litter of seven pups. I took Ick! away
permanently. Then I isolated the mother and her babies by
taking her siblings away and leaving her alone with her
babies in the cage. I left them alone for the night. In the
morning, I looked for the babies and found that each one had
been methodically bitten once in the head and killed.

I put all the rats except for Ick! back in the same cage.
Two days later, on Dec 30, a second rat "boy" had a litter
of five; I thought I would do it differently and left them
all together, but the next morning each of these were also
found bitten once in the head. On January 12, 2000, a third
rat "boy" had a litter of three; these babies were ignored
and soon died.

Note that the babies were all born alive, and moved and
squeaked loudly. I did not observe if they nursed at
all.

I was very confused. These animals had been identified by
the rehab center and my vet at Animal Medical Center, NYC,
as wild Norway boys. How could they make such a mistake?

The mystery continued until a few months later when a
friend sent me a photograph of his Rattus rattus
girl. To my astonishment, his rat looked exactly like the
four "boys". I sought out more information, identified the
four "Norway boys" to actually be four R. rattus
girls and learned these relevant facts about R.
rattus (From Claire
Jordan's Rattus Website) -- The female ship/roof rat has
a genital mound that makes it difficult to sex them at an
early age. In addition, R. rattus has the physical
aspect of a mouse and are petite and better climbers (almost
"arboreal") compared to R. norvegicus. This explained
why the "boys" never seemed to mature.

Jane Adamo

July 28, 2003

Interspecies
hybridization

Avoiding the wrong species:
reproductive isolating mechanisms

The overwhelming majority of species cannot and do not mate with
each other. Crocodiles and horses, butterflies and falcons,
earthworms and fleas, jellyfish and tuna and hundreds of thousands of
other combinations of species cannot and do not interbreed with each
other.

In a few rare instances, however, very closely related species may
have the potential to interbreed. Horses, donkeys and zebras
may interbreed, for example, as may lions and tigers, or bison and
cattle.

In these cases it is nevertheless maladaptive for one species to
mate with another. Mating with the wrong species is a waste of time
and energy. Such matings are usually unsuccessful: the hybrid
offspring die or are sterile, so all the reproductive effort of the
parents is ultimately wasted.

There are a number of mechanisms which prevent individuals from
mating with the wrong species. These are called reproductive
isolating mechanisms:

Physical separation: species who live in different
geographic locations or occupy different ecological niches in the
same location never have the chance to meet each other. In the
wild, lions live in Africa and tigers in India, so there is no
chance for them to mate. Two species may occupy different
ecological niches and thus not mate with each other, for example,
one species may lives in the jungle canopy and another may live on
the jungle floor.

Temporal isolation: Species that mate during different
seasons or time of day cannot breed together. For example, ants
mate on one day of the year: each colony sends up swarms of
reproductive males and females. They mate and the females start
colonies of their own. In the American Southwest, different ant
species generally perform their mating flights on different days,
thus preventing accidental matings between species.

Behavioral isolation: members of different species may
meet each other, but do not mate because neither performs the
correct mating ritual. Elaborate mating rituals play an important
role in species recognition. For example, tungara frog females
prefer the calls of their own males (Kirkpatrick and Ryan 1991).
Male zebra finches imprint on their mothers and seek out and court
only females who look like their mom (Bischof 1994).

Mechanical isolation: Copulation may be impossible
because of incompatible size and shape of their reproductive
organs.

Morphological isolation: Copulation may be impossible
because of the difference in body size or shape.

Gametic isolation: Even if copulation occurs, the sperm
and egg do not fuse -- fertilization cannot occur (e.g. rats
and mice)

Do different
species ever mate with each other?

The reproductive isolating mechanisms described above are not
always foolproof. In captivity, a human can intervene and create
conditions that lead to an animal mating with another of a different
species. For example, raise a baby male zebra finch with a Bengalese
mother finch, and the zebra finch will grow up to ignore female zebra
finches but devotedly court Bengalese finches (Bischof 1994).

Rarely, some crosses may happen naturally in the wild between very
closely related species.

What happens if an
interspecies mating takes place?

Interspecies mating usually fails, but it can fail at many
different points after fertilization. These failures are called
postzygotic reproductive isolating mechanisms, because they isolate
one species from another even after fertilization has occured
(postzygotic means "after fertilization").

The more related the species are to each other, the later in the
reproduction process the mating tends to fail. Hybrids may die before
or after implantation, during the pregnancy, or around the time of
birth, For example, Norway rats and roof rats may produce non-viable
fetuses or infants.

Species that are even more closely related to each other may
produce viable but sterile offspring. For example, horses and donkeys
produce sterile mules. Extremely closely related species may produce
offspring that are partially fertile. For example, domestic cats and
their wild relatives, and cattle and bison, tend to produce fertile
female hybrids but sterile male hybrids.

Lastly, two species may be so closely related that they produce
fertile offspring. For example, dogs and wolves produce fertile
hybrids. Interspecies matings that produce fertile offspring
challenge our notion of the biological species concept.

There is individual variation in interspecies matings, too. Not
all matings between one species and another will fail at exactly the
same point every time. For example, reproduction between goats and
sheep almost always fails, but there is one recorded case of a
viable, sterile goat-sheep hybrid. Similarly, Norway rats and roof
rats tend not to produce offspring (fertilization does not occur or
the embryos die in utero) but in a few cases hybrid offspring have
been produced that died shortly after birth. As another example,
horses and donkeys usually produce sterile mules, but in a few
instances a female mule may be fertile. Because of this individual
variation, it makes more sense to talk about success or failure
rates: what percentage of matings (if any) between species A
and B produce offspring? If offspring are produced, what percentage
(if any) are fertile?

Hybrid inviability: Embryo death or
stillbirth

If the species are related, fertilization may occur but the
embryo dies. The miscarriage may happen very early, such that
the fertilized egg fails to implant in the uterus, or it may happen
at some point during the pregnancy. Lastly, the offspring may be
brought to term but the offspring may be stillborn, dying
before, during or shortly after birth.

Matings between these species do not produce viable
offspring:

Water buffalo and cattle: Water buffalo (Bubalus
bubalis) and cattle (Bos taurus) embryos fail around
the 8-cell stage (Patil and Totey 2003).

Rats: Norway rat (Rattus norvegicus) and roof
rat (Rattus rattus) matings are usually unsuccessful, but
on a few occasions live offspring have been produced which died
shortly after birth.

Two species may produce viable offspring which may survive
to adulthood. Such hybrids are usually sterile. In some cases,
however, some of the hybrid offspring may be fertile as
adults. Generally, female hybrids are more likely to be
fertile than males.

Matings between these species usually fail, but a few viable,
sterile offspring are on record:

Goats and sheep: Goats and sheep tend not to produce
offspring, but in a single case a sterile goat-sheep
hybrid was produced in Botswana.

Matings between these species tend to produce viable, sterile
offspring:

Lovebirds: Fischer's lovebirds (Agapornispersonata fischeri) and Peach-faced lovebirds (Agapornis
rosecollis) can interbreed and produce sterile offspring.
Interestingly, the lovebird hybrid displays intermediate nesting
behavior. Fisher's lovebirds carry single strips of nest material
in their beaks. Peach-faced lovebirds tuck many pieces of nest
material between their rump feathers. The hybrids show a poorly
organized mixture
of the two strategies: they tuck nest material between their
feathers but fail to let go, pull it out again, and start over.
After several months, they can become partly successful, managing
to transport some material back to the next site, but not in a
manner that resembles either parent species. Sometimes they just
turn their heads toward their rumps without tucking, then fly off
with the material (Dilger, 1962).

Lion-leopard crosses produce viable but sterile
offspring called leopons.
The most famous leopons were five cubs born in Hanshin Park in
Japan (two in 1959, three in 1962). The last one died in 1985 (Doi
and Reynolds 1967).

Matings between these species produce viable offspring that are
usually sterile, but a few fertile female hybrids are on
record:

Whales: Several naturally occurring crosses between
blue whales (Balaenoptera musculus) and fin whales
(Balaenoptera physalus) have been identified. One male
hybrid was sterile, and of the two female hybrids, one was sterile
while the other was pregnant, though it is unknown whether her
fetus would have been viable (Arnason et al. 1991,
Spillaert et al. 1991, Bérubé and Aguilar
1998).

Dolphin and false killer whale: There has been one case
of a female bottlenose dolphin (Tursiops truncatus) and a
male false killer whale (Pseudorca crassidens) producing a
fertile female hybrid, called a wolphin.
She went on to breed with a dolphin and produced a daughter.

Matings between these species produce hybrids of unknown
fertility:

Bobcat and lynx: Bobcats (Lynx rufus) and lynxes
(Lynx canadensis) may cross; several such crosses have
happened naturally in the wild
(also Q&A
on Minnesota bobcat-lynx crosses (pdf)).

Porpoises: Dall's porpoises (Phocoenoides dalli)
and harbour porpoises (Phocoena phocoena) can conceive
offspring. Many individuals with intermediate pigmentation
have been observed, indicating that such offspring may be viable
(Baird et al 1998).

Viable, fertile offspring

If the parent species are extremely closely related, they may
producefertile offspring. These are the edge cases in
which the biological species concept can become too rigid. The
biological species concept states that animals belong to a separate
species if they cannot interbreed. So, if animals supposedly from
different species interbreed and produce fertile offspring, then
according to the biological species concept they should be one
species. However, we usually don't consider them the same species
because they differ in other features, such as geographic location,
appearance, behavior, and genetics.

Evidence for red wolf-coyote hybridization (Adams et
al. 2003a); Northeastern
coyotes may be the product of hybridization between
Canadian wolves and Western coyotes.

Cichlid species in Lake Victoria are extremely
variable, displaying 500 color morphs. The species are isolated
through mate choice, which is determined by coloration: mates
choose each other by color pattern. However, in recent years,
human activity has caused the water of Lake Victoria to become
cloudy. In these areas, the cichlids can't differentiate between
species. In these cloudy areas, bright color morphs have
disappeared and the fish have become similar and dull in
appearance through hybridization
(Seehausen et al. 1997).